A number of transducer, exciter or actuator mechanisms have been developed to apply a force to a structure, e.g. an acoustic radiator of a loudspeaker. There are various types of these transducer mechanisms, for example moving coil, moving magnet, piezoelectric or magnetostrictive types. Typically, electrodynamic speakers using coil and magnet type transducers lose 99% of their input energy to heat whereas a piezoelectric transducer may lose as little as 1%. Thus, piezoelectric transducers are popular because of their high efficiency.
There are several problems with piezoelectric transducers, for example, they are inherently very stiff, for example comparable to brass foil, and are thus difficult to match to an acoustic radiator, especially to the air. Raising the stiffness of the transducer moves the fundamental resonant mode to a higher frequency. Thus such piezoelectric transducers may be considered to have two operating ranges. The first operating range is below the fundamental resonance of the transducer. This is the “stiffness controlled” range where velocity rises with frequency and the output response usually needs equalisation. This leads to a loss in available efficiency. The second range is the resonance range beyond the stiffness range, which is generally avoided because the resonances are rather fierce.
Moreover, general teaching is to suppress resonances in a transducer, and thus piezoelectric transducers are generally used only used in the frequency range below or at the fundamental resonance of the transducer. Where piezoelectric transducers are used above the fundamental resonance frequency it is necessary to apply damping to suppress resonance peaks.
The problems associated with piezoelectric transducers similarly apply to transducers comprising other “smart” materials, i.e. magnetostrictive, electrostrictive, and electret type materials.
It is known from EP 0 711 096 A1 of Shinsei Corporation to provide a sound generating device in which a driving device of an acoustic vibration plate is arranged between a speaker frame and the acoustic vibration plate. The driving device is comprised of a pair of piezoelectric vibration plates arranged facing each other across a certain distance. The outer peripheries of the piezoelectric vibration plates are connected to each other by an annular spacer. When a drive signal is applied to the piezoelectric vibration plates, the piezoelectric vibration plates repeatedly undergo flexing motion wherein their centres flex alternately in opposite directions. The flexing directions of the piezoelectric vibration plates are always reverse to each other.
It is known from EP 0881 856A of Shinsei Corporation to provide an acoustic piezoelectric vibrator and loudspeaker using the same, wherein an oscillation controlling piece of elastomer is attached to the periphery of a piezoelectric oscillation plate. The oscillation controlling piece is shaped so that a distance between an axis passing by a centre of the piezoelectric oscillation plate, which is perpendicular to a straight line connecting a centre of the piezoelectric oscillation plate to the centre of gravity of the oscillation controlling piece, and a mass centre line of the oscillation controlling piece varies along the axis, or so that a mass of each of sections of the oscillation controlling piece divided by a plurality of straight lines parallel to a straight line connecting a centre of the piezoelectric oscillation plate to the centre of gravity of the oscillation controlling piece varies along an axis which is perpendicular to the straight line and passes through the centre of the piezoelectric oscillation plate.
U.S. Pat. No. 4,593,160 OF Murata Manufacturing Co. Limited discloses a piezoelectric speaker comprising a piezoelectric vibrator for vibrating in a bending mode, which is supported at its longitudinal intermediate position by a support member, whereby first and second portions of the piezoelectric vibrator on both sides of the support member are respectively supported in a cantilever manner. The piezoelectric vibrator is connected at portions close to both ends thereof with a diaphragm by coupling members formed by wires, whereby bending vibration of the piezoelectric vibrator is transferred to the diaphragm thereby to drive the diaphragm. The position of the support member with respect to the piezoelectric vibrator is so selected that the resonance frequency of the first portion is smaller than the corresponding resonance frequency of the second portion, and the primary resonance frequency (f1) of the second portion is so selected as to be substantially at the centre value of the first resonance frequency (F1) and the second resonance frequency (F2) of the first portion on logarithmic coordinates.
U.S. Pat. No. 4,401,857 of Sanyo Electric Co Limited discloses a piezoelectric cone-type speaker having a multiple structure in which a plurality of piezoelectric elements and speaker diaphragms individually coupled to them are coaxially or multi-axially arranged. A cushioning member is interposed between one diaphragm and another so that each element is isolated from the vibrations of another element.
U.S. Pat. No. 4,481,663 of Altec Corporation discloses a network for matching an electrical source of audio signals to a piezoceramic driver for a high frequency loudspeaker. The network consists of all of the elements of a bandpass filter network, but with the parallel combination of an inductor and a capacitor in the output stage of the filter replaced by an autotransformer or autoinductor which transforms the input impedance of the piezoceramic transducer into an equivalent parallel capacitance and resistance which, together with the inductance of the autotransformer, supply the load resistance for the filter and replace the capacitor and inductor omitted from the output stage of the bandpass network. An additional shunt resistor may be placed across the output of the autotransformer to obtain the desired effective load resistance at the input of the autotransformer.
UK patent application GB 2,166,022A of Sawafuji discloses a piezoelectric speaker including a plurality of piezoelectric vibrating elements, each including a piezoelectric vibrating plate and a weight connected to the plate near the point of centre of gravity thereof through a viscoelastic layer, and having the vibramotive force designed to be taken out of the outer edge thereof. The piezoelectric vibrating elements are connected at their peripheral ends to each other through connectors, one of the elements being connected at its peripheral edge directly to a cone type acoustic radiator to give the radiator a vibramotive force mainly in a high-frequency portion, and the remaining elements adjacent thereto producing a vibramotive force adapted to share middle- and low-frequency portions for energization of the cone type acoustic radiator.
It is an object of the present invention to provide an improved transducer.